1. Field of the Invention
This invention relates to a microwave dividing and combining waveguide to be used in, for example, microwave communications.
2. Description of the Related Art
In microwave communications, various types of microwave dividing and combining waveguides are currently known.
FIG. 8 of the accompanying drawings shows a typical conventional waveguide. The conventional waveguide generally comprises a common port 90, a pair of distribution ports 91a, 91b, and a pair of quarter-wavelength impedance transformers 92a, 92b located between the distribution ports 91a, 91b.
The microwaves inputted from the common port 90 are divided by the two quarter-wavelength impedance transformers 92a, 92b and are outputted from the two distribution ports 91a, 91b. Reversely, the microwaves of the phase inputted from the two distribution ports 91a, 91b are combined by the two quarter-wavelength transformers 92a, 92b and are outputted from the common port 90.
This type of waveguide is disclosed in S. B. Cohn "A class of Broad Band Three-part TEM-Mode Hybrid" IEEE Transmission Microwave Theory Technology, MIT-16, No. 2 Feb. 1968), p.p. 110-116.
Recently a microstrip line has been developed to reduce the entire waveguide in size and widely utilized in microwave area.
In this microstrip line, as shown in FIG. 9, a strip conductor 96a is located on one surface of a dielectric substrate 95a, and a ground conductor 97a is located on the other surface of the dielectric substrate 95a.
Since linearly extending quarter-wavelength impedance transformers are used, the first-named conventional waveguide is difficult to reduce in size.
Also in the second-named waveguide in the form of a microstrip line, the width of the strip conductor 96a is determined from the dielectric constant and thickness of the dielectric substrate 95a; since the degree of freedom in designing is relatively small, there is a restriction in reducing the size of the waveguide.
In some of those types of conventional waveguides, resistors (isolation resistors) are provided to adjust the phase difference produced in the two impedance transformers. For example, as shown in FIG. 8, a plurality of isolation resistors 98 are located at a number of positions between the two strip conductors 92a, 92b. These isolation resistors 98 connect the two strip conductors 92a, 92b with each other at positions thereof to absorb the microwaves which are different in phase from each other.
However, if isolation resistors were used in the waveguide in the form of a microstrip line, a divided capacity would have occurred between the isolation resistors and the ground conductors of the microstrip line. Because of this divided capacity, the characteristic impedance of the individual impedance transformer will not reach 50.OMEGA. and will thus adversely affect the voltage standing wave ratio characteristics.
To this end, a solution has been proposed, as disclosed in Japanese Patent Laid-Open Publication No. Sho 63-246002, in which the capacitive component such as a condenser is located at a branch point of one end of the line serving as the impedance transformer. This construction, however, requires more circuit elements, making the line structure complex.